1. Field of the Invention
The present invention relates to low power density radio systems. In particular, the invention relates to systems suitable for use in the first hop between a meter transponder and a receiver in an automatic meter reading (AMR) system. The system is also suitable for other applications.
2. The Prior Art
It is difficult to find unused radio channels throughout the world. This is particularly true for low power, short range devices (SRD), which operate under license exemption. In many countries, particularly in the United States, the licensing authorities have encouraged the use of various types of spread spectrum modulation devices to allow for the uncoordinated sharing of channels. The normal methods of reducing the power density, and hence interference potential on any channel is to use either frequency hopping (FH), or direct sequence spread spectrum (DSS).
In frequency hopping, the transmitter skips around a series of randomly selected narrow band channels and spends only a short time on any one frequency. For example, FCC Part 15.247 specifies the use of a minimum of 50 channels, with a maximum time of 0.4 seconds spent on any one channel in a 20 second period. In this way, the average power density on any channel from a given transmitter is reduced to 2% of the power that would be present if it was not channel hopping. The alternative method, DSS, requires that the narrow band carrier be modulated by a high speed pseudo-random sequence, which has the effect of spreading the signal over a wide bandwidth. Again, in FCC Part 15.247, the spreading gain is specified as a minimum of 20 dB with a minimum bandwidth of 500 kHz. Thus, the average power density of a given transmitter is reduced to 1% or less.
AMR is a low power, low cost radio system that allows utility meters to be read from receivers that are hand held or in mobile computers. A radio device, normally referred to as a transponder, is attached to the meter, so that it can either count pulses or access the meter's internal registers. The transponder is then interrogated, i.e., activated by the reader, so as to send the desired information to the reader. The transponder will then be deactivated until the next interrogation sequence. The major problem with this approach is the high cost, both in terms of the components used and power consumed by the receiver in the transponder.
Low power density is particularly significant for AMR since the meter transponder is required to be a low maintenance and reliable device. For electric meters, the power supplied is not a problem. However, for transponders used with water, gas or other non-electric utilities, it is generally necessary for the transponder to be battery powered. This requires that continuous power consumption be kept in the 10-20 .mu.A range. A low power density operation therefore enables long battery life.
The approach taken by most AMR systems for non-electric meters is to provide a transponder that responds to a wake-up signal and can therefore be powered down for long periods of time. The use of a wake-up signal requires the transponder to have a receiver. This considerably increases the cost and complexity of the transponder. Prior art solutions also use spread spectrum devices to achieve low power density.